1. Field of the Invention
The present invention refers to a drive circuit for controlled edge power elements.
2. Description of the Related Art
In the drive circuit of the power elements, the leading and trailing edges of the signals to apply to the load are kept under control, so as to reduce the width of the components in frequency on the load. In particular leading and trailing edges having limited slopes are forced, that is steep edges rich with harmonics are avoided. An integrator is generally used to control the edges by means of a suitable gain stage that drives a power element typically a MOS transistor.
With the reduction of the construction geometries of integrated circuits, the situation is presented in which the breaking stress of the gate oxide of the MOS transistors is lower than the supply voltage of the circuit in which the transistors are used. In this case, in order to avoid the breaking of the gate oxide, it is necessary to limit the voltage variation thereacross coming from the integrator. To limit the output voltage of the integrator one is forced to use voltage clipping circuits that often present inconveniences and complicate the circuit.
In the case of controlled edge halfbridges two drive circuits are necessary, an upper and a lower one. In the upper drive circuit the MOS transistor acts as source follower, while in the lower drive circuit the MOS transistor acts as open drain. The result is that the behavior of the two drive circuits is different and thus the two circuits have to be optimized separately.
In addition, in this case the two circuits are never active contemporarily to avoid phenomena of crossconduction, and the Applicant has perceived that it could reduce the area of silicon occupied if several components were shared between the two circuits.
We now refer to FIG. 1, which shows a drive circuit for controlled edge power elements of the high-side type according to the known art. It comprises a first power element which in FIG. 1 is for example represented by a MOS transistor M1 having the drain connected to a supply voltage Vcc and the source connected to a terminal of a load L, which in turn is connected to ground Gnd. The gate of the transistor M1 is driven by an operational amplifier OP1, which has non-inverting input connected to a reference voltage Vref and an inverting input connected to a terminal of a capacitor C1 and a first terminal of a switch SW1. The other terminal of the capacitor C1 is connected to the load L. The switch SW1 permits, on command of a special circuit not shown in the figure, the commutation of said first terminal between a second and a third terminal. The second terminal is connected to a current generator IH which in turn is connected to the supply voltage Vcc. The third terminal is connected to a current generator IL which in turn is connected to ground Gnd. The generator IH supplies current to the capacitor C1, while the generator IL extracts current from the capacitor C1.
The drive circuit therefore comprises an integrating circuit comprising an operational amplifier OP1 and the capacitor C1, which withdrawing the voltage on the load L and having a charge and discharge current determined by the current generators IH and IL permits the slope of the leading and trailing edges applied to load L to be controlled.
We now refer to FIG. 2 which shows a drive circuit for controlled edge power elements of the halfbridge type according to the known art. It comprises all the circuit elements shown in FIG. 1 that keep the same numerical references, with exception of the L, which in this case has one terminal (Out) connected to transistor M1 and the other terminal is not shown in the figure and can be connected to ground Gnd or to another halfbridge like that represented here. It comprises, in addition, a second power element (low-side) which in the figure is for example represented by a MOS transistor M2 having the drain connected to the terminal of the load L and to the source of the transistor M1, the source of the transistor M2 is connected to ground Gnd. In alternative the source could be connected to a negative voltage xe2x88x92Vcc. The gate of the transistor M2 is driven by an operational amplifier OP2 which has the inverting input connected to a reference voltage Vref and a non-inverting input connected to a terminal of a capacitor C2 and a first terminal of a switch SW2. The other terminal of the capacitor C2 is connected to the load L. The switch SW2 permits, upon command from a special circuit which is not shown in the figure, to commutate said first terminal between a second and a third terminal. The second terminal is connected to a current generator IH2 which in turn is connected to the supply voltage Vcc. The third terminal is connected to a current generator IL2 which in turn is connected to ground Gnd. The generator IH2 supplies current to the capacitor C2, while the generator IL2 extracts a current from the capacitor C2.
It can be noted that in the upper drive circuit the transistor M1 acts as source follower, while in the lower drive circuit the transistor M2 acts as open drain.
An embodiment of the present invention provides a drive circuit for controlled edge power elements that does not have the inconveniences of the known art.
The drive circuit comprises: a first integrating circuit having a first input suitable for receiving in input a first drive signal; an integrating capacitor coupled to said integrating circuit; a first power element driven by said first integrating circuit and suitable for driving a load, said load having a first terminal; characterized in that said first integrating circuit includes a first current amplifier and that said integrating capacitor is coupled between said first input and a predetermined reference voltage.
Thanks to the present invention, not coupling the integrating capacitor directly to the load, it is possible to prevent the recirculation voltages under-ground and over-voltage present on the load from activating parasitic effects depending on the construction of the capacitor on silicon; and in addition an open drain type drive circuit is obtained which permits the division of the voltage gain between the current amplifier and the power element.
In addition thanks to the present invention drive circuits for controlled edge power elements can be realized that eliminate the use of clipping circuits. There is the possibility of using low voltage components for the drive of the power elements and in particular CMOS components which are less sensitive to the problems of under-ground and over-voltage typical of the drive stages of inductive loads such as motors and solenoids. In addition, in the case of controlled edge halfbridges symmetrical drive circuits are obtained, and only one integrating capacitance is used saving considerable space on the silicon.